This project explores the fate of CD8+ effector T cells in patients with cancer. Our data show that tumor-infiltrating as well as circulating lymphocytes are susceptible to spontaneous apoptosis in these patients. CD8+ T cells which are particularly susceptible, bind Annexin V and/or have a low level of the T-cell receptor-associated zeta chain. Decreased T-cell numbers and dysfunction of T cells are common findings in patients with head and neck cancer (HNC) and melanoma. We will test the hypothesis that in patients with these cancers, tumor-specific CD8+ T cells are preferentially targeted for apoptosis, which disrupts normal lymphocyte homeostasis and leads to dysregulated anti-tumor responses. In Aim 1, a clinical trial will be performed to in vivo measure absolute production rates of CD4+ and CD8+ cells, their half-life and survival time in the circulation of patients with HNC, using deuterated drinking water to label DMA in these lymphocytes. In addition, TREC and telomere length assays will be performed in a larger cohort of HNC patients to corroborate a rapid T-cell turnover. In Aim 2, we will explore the preferential demise of CD8+ effector T cells by a combined use of T-cell surface and apoptosis biomarkers in multi-color flow cytometry. Various T-cell subsets, including tetramer+ tumor-specific T cells in the peripheral circulation of patients with HNC or melanoma will be targeted. Aim 3 will evaluate mechanisms responsible for CD8+ T-cell demise by focusing on the selected molecular targets in the Fas/FasL or mitochondria! pathways of apoptosis. Elimination of CD8+ T cells via a novel mechanism involving circulating membraneous vesicles (MV) will be studied. In Aim 4, effects of the cytokines known to regulate T-cell homeostasis (IL-2, IL-7, IL-12 and IL-15) on the molecular targets engaged in apoptosis will be evaluated as will mechanisms responsible for the protective effects on T cells of the anti-apoptotic protein Mcl-1. In all Aims, efforts will be made to relate expression and function of immune biomarkers studied in CD8+ T cells with the presence and activity of the disease and with the known prognostic factors for the disease. These studies will serve as a basis for the development of novel therapies designed to deliver survival factors to patients with cancer in order to rescue tumor-specific T cells from apoptosis, normalize homeostasis and amplify anti-tumor immune responses.